In 2.4 MongoDB includes two new features related to indexes that users
upgrading to version 2.4 must consider, particularly with regard to
possible downgrade paths. For more information on downgrades, see
Downgrade MongoDB from 2.4 to Previous Versions.

In 2.4 MongoDB adds two new index types: 2dsphere and
text. These index types do not exist in 2.2, and for each
database, creating a 2dsphere or text index, will upgrade the
data-file version and make that database incompatible with 2.2.

If you intend to downgrade, you should always drop all 2dsphere
and text indexes before moving to 2.2.

You can use the downgrade procedure to downgrade these
databases and run 2.2 if needed, however this will run a full database
repair (as with repairDatabase) for all affected
databases.

In MongoDB 2.2 and earlier you could specify invalid index types that
did not exist. In these situations, MongoDB would create an ascending
(e.g. 1) index. Invalid indexes include index types specified by
strings that do not refer to an existing index type, and all numbers
other than 1 and -1. [1]

In 2.4, creating any invalid index will result in an error.
Furthermore, you cannot create a 2dsphere or text index on a
collection if its containing database has any invalid index types.
[1]

Example

If you attempt to add an invalid index in MongoDB 2.4, as in the
following:

(1, 2) In 2.4, indexes that specify a type of
"1" or "-1" (the strings "1" and "-1") will continue
to exist, despite a warning on start-up. However, a
secondary in a replica set cannot complete an initial sync
from a primary that has a "1" or "-1" index. Avoid all
indexes with invalid types.